Fix painting a grid item overflowing its grid area

https://chromiumcodereview.appspot.com/22642010 wasn't totally
correct and we ended up not properly painting a grid item when
it overflowed its grid area. This is a quick fix that disable
the optimization in this case and includes some testing if we
decide to be smarter about it.

BUG=248151

Review URL: https://codereview.chromium.org/23439008

git-svn-id: svn://svn.chromium.org/blink/trunk@158595 bbb929c8-8fbe-4397-9dbb-9b2b20218538